Despite being among the most overlooked aspects of an aircraft avionics system, the antenna is perhaps the most important. Aside from certain devices, like autopilots, avionics rely heavily on antennas to communicate. Modern antennas are available in many different shapes and sizes, each formed by its function. More often than not, a well-equipped aircraft will feature a collection of antennas, making it difficult to understand what each antenna does. However, when you look at them one by one, aircraft antennas are easy to understand. Their size, shape, and placement is generally determined by the frequencies at which they are used and their directional qualities. This blog will cover the most common types of antennas found on aircraft as well as their functions.
The first type of antenna is the communication antenna. These are relatively basic in their operation. For redundancy, each communications transmitter is equipped with its own antenna. Communication antennas can be mounted on the top or bottom of the aircraft, but either way, they are susceptible to shadowing from the fuselage. Shadowing refers to interference caused by structures like the vertical stabilizer or landing gear doors being in the path of the antenna. The placing of your antennas and how shadowing may affect their range and coverage is important to note.
The next antenna type, UHF (ultra high frequency), is frequently used for transponders and distance measuring equipment (DME) and is always located on the bottom of the aircraft. UHF antennas are approximately four inches in length, and a single antenna can be used for both the transponder and DME system because the transponder’s frequency is within that of the DME. two types of UHF antennas are used: spike and blade. Spike antennas should only be used for transponders because the antenna is only tuned to the transponder frequency. The blade antenna is tuned to a range of DME frequencies. As such, blade antennas are also known as broadband antennas. A spike antenna would not work for a DME - the blade antennas have a better radiation pattern.
Spike antennas are also prone to oil buildup which reduces their transmission range. Keeping them clean can double your transponder range and eliminate transmission problems. Blade antennas are not perfect either; they are prone to delamination which detunes the frequency response and can distort the transmitted signal. For these reasons, regular transponders checks are critical.
The VHF (very high frequency) antenna is nearly always on the vertical tail of the aircraft. There are three types of such antenna: the cat whisker, dual blade, and towel bar. A cat whisker comprises a couple of rods extending from each side of the vertical stabilizer at a 45-degree angle. Despite this, cat whisker antennas struggle to receive signals from the side. The dual blade, as its name suggests, consists of two blades, one on each side of the tail. Finally, the towel bar antennas, which look like the common bathroom fixture, sit on each side of the tail. The blade and towel bar antennas have equal receiving sensitivity from all sides, making them more attractive options than the cat whisker. There is usually only a single nav antenna feeding multiple nav receivers, making it a critical piece of equipment. If the nav antenna fails, multiple systems could potentially malfunction.
GPS antennas, which are mounted on top of the fuselage, transmit less than five watts of power, making their signals very weak. For this reason, GPS antennas have built-in amplifiers to bolster the signal. Furthermore, GPS frequency is so high (in GHz) that the signal travels in a line-of-sight manner. This makes it vulnerable to shadowing, which is why it must be mounted atop the fuselage. Communications radios can cause a lot of interference with GPS, making it critical that communication and GPS antennas are located as far away as possible from each other. To avoid interference, comm antennas are often moved to the bottom of the aircraft.
Marker Beacon Antennas
Signals from market beacons are highly directional, meaning the aircraft must be nearly directly above the transmitting ground station to receive them. As such, marker beacons must be located on the bottom of the aircraft. While there are multiple types of marker beacon antennas, the most common type resembles a canoe and is about ten inches long. In certain installations, Cessna has used a marker beacon antenna that appears to be flush, featuring flat plates under the empennage. Cessna has also used an antenna that features a thick wire that extends downward out of the empennage and turned toward the tail.
Emergency Locator Transmitter Antennas
Ideally, you will never have to use this antenna. But, in the event you do, the emergency locator transmitter antenna is designed to survive an unscheduled landing. These are most commonly located on the upper skin of the empennage and made from a flexible material. In certain configurations, however, they may be inside the vertical tail or look like small communication antennas.
Finally, there are many important performance considerations to make. For one, the physical condition of an antenna plays a crucial role in its performance. If the antenna is cracked, for example, water can enter and cause delamination which renders the antenna useless. Furthermore, if the base of the antenna is structurally weak, the antenna will vibrate more, causing the skin to fatigue and leading to cracks. The antennas must also be electrically bonded to the airframe to allow for a strong electrical connection. If an antenna becomes corroded, this bond will be weakened and cause the antenna’s efficiency to suffer. Whatever type of antenna you have, maintaining its condition is critical.
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